Syphilis is a global public health problem that results in significant morbidity and infant mortality. The continued worldwide prevalence of adult and congenital syphilis, and the increased risk of transmitting HIV infection in patients co-infected with syphilis, underscores the need for a safe and effective syphilis vaccine. The focus of this proposal is the isolation of rabbit monoclonal antibodies directed against Treponema pallidum surface antigens that are targets of treponemicidal activity. We believe that such monoclonal antibodies and the identification of their respective targets would facilitate the development of an experimental syphilis vaccine. Previous studies aimed at identifying elements of protective immunity in syphilis have sought to characterize the surface of T. pallidum on the basis that surface proteins of the spirochete were certain to be involved in pathogenesis and to constitute targets of protective immunity, as in the case of other microbial pathogens. An approach we previously employed to characterize the T. pallidum surface was based upon our finding that immunization of mice with purified T. pallidum outer membrane vesicles (OMV) induced remarkably high titer treponemicidal antibodies. It is pertinent to note that subsequent efforts to immunize rabbits with OMV did not generate treponemicidal antibodies in spite of a good immune response to outer membrane associated lipoproteins as determined by Western blot analysis. A monoclonal antibody, termed M131, was identified with potent in vitro treponemicidal activity. M131 binds a phosphorylcholine epitope on the T. pallidum surface. Because phosphatidylcholine contains phosphorylcholine, which is a constituent of mammalian membranes, future vaccine development based upon this antigen would be unacceptable due to the induction of autoimmunity in the host. The aim of this study is to utilize a similar strategy by generating monoclonal antibodies from the spleen of a syphilitic rabbit immune to challenge reinfection, in order to identify more promising T. pallidum determinants of killing antibody and protective immunity. The development of a human vaccine represents the ultimate long-term objective of future studies. PUBLIC HEALTH RELEVANCE: Syphilis is a global public health problem that results in significant morbidity and infant mortality. The continued worldwide prevalence of adult and congenital syphilis, and the increased risk of transmitting HIV infection in patients co-infected with syphilis, underscores the need for a safe and effective syphilis vaccine.